1. Field of the Invention
The present invention relates to an awakening level estimation apparatus and an awakening level estimation method for vehicle, and particularly to a technique for estimating an awakening level of a driver by monitoring a displacement of a vehicle in a direction of vehicle width in a time series manner.
2. Description of the Related Art
Development of a technique for preventing an accident caused by a decrease in an awakening level of a driver is one of important study problems from the viewpoint of safety, and studies on a technique for detecting a decrease in an awakening level or a warning art have been conducted actively. An awakening level estimation technique capable of accurately deciding an awakening level even in case that a large change in travel environment or vehicle speed occurs is disclosed in a JP-A-2002-154345 which is prior application of an applicant of the present application. In this estimation technique, displacement amounts of a vehicle in a direction of vehicle width is first detected in a time series manner and each frequency component power is calculated by making frequency conversion of these displacement amounts. Next, an average value of each of the frequency component powers is calculated as a high frequency component amount. Together with that, a maximum value of the frequency component powers within a predetermined frequency domain including a stagger frequency to become apparent in a state in which an awakening level of a driver decreases is calculated as a low frequency component amount. Then, an awakening level of a driver is decided based on an evaluation value corresponding to a ratio of the high frequency component amount to the low frequency component amount.
In the conventional art described above, it is decided that the awakening level of the driver is low in the case that the high frequency component amount is small and the low frequency component amount is large. However, there is a personal difference among drivers in the high frequency component amount and the low frequency component amount resulting in a criterion of awakening level estimation. As a result of that, there is a possibility that an accurate decision on the awakening level becomes difficult in the case that both of these component amounts are large (a driver with a large stagger) or the case that both of these component amounts are small (a driver with a small stagger).